1. Field of the Invention
The present invention relates to a polyethylene enjoying outstanding electrical properties even in a high electric field and adapted for use as a material of an insulation layer of a power cable, and a crosslinked polyethylene insulated power cable using the same.
2. Prior Art
Power cables include OF (oil-filled) cables, which use an insulation layer formed of kraft paper or plastic film laminated paper impregnated with insulating oil, and crosslinked polyethylene insulated power cables whose insulation layer is formed of a crosslinked polyethylene.
Among these power cables, the crosslinked polyethylene insulated power cables have come to be widely used, owing to their easy maintenance and smaller dielectric loss. As the fields of their application expand, the working environment advances toward use of higher-voltage classes.
With the progress of the higher-voltage classes of these crosslinked polyethylene insulated power cables, the intensity level of the mean operating electric field rises, and the operating cable temperature becomes higher.
In the case of a crosslinked polyethylene insulated power cable of the 275-kV class, for example, the mean operating electric field is about 6 kV/mm, and the operating temperature becomes as high as about 90.degree. C.
As the operating electric field increases in this manner, the field concentration on defective portions in the insulation layer grows larger than ever, and the field intensity may reach scores of kilovolts per millimeter, in some cases.
When the insulation layer of the crosslinked polyethylene is placed in a high-temperature, high electric field, the dissipation factor (tan.delta.) increases to produce heat in the insulation layer, so that the dielectric breakdown voltage may lower considerably. Also, the dielectric loss is augmented with the increase of the tan.delta., so that the transmission capacity of the power cable may decrease.
In order to maintain high insulating performance for the insulation layer for a long period of time, the tan.delta. must be kept on a low level even if the applied electric field is about twice as high as the mean operating electric field.
For the manufacture of a crosslinked polyethylene insulated power cable with stable insulating performance and large transmission capacity, therefore, it is necessary to form an insulation layer of a crosslinked polyethylene whose tan.delta. can be kept low even in a high mean operating electric field.